C11 Mechanical Alignment & Collimation

Mirror Modifications - Mirror Bond

A stainless steel flange was machined to a tight fit to the inner bore in the glass of the primary mirror and bonded to it. There is no steel to glass contact. The flange gap between steel and mirror inside the mirror hole is only 1-2mm. The glass ID is not perfectly cylindrical all the way. The mirror is only bonded with RTV on half of its length.

*note: the back of the mirror glass is free and not bonded to the face of the stainless steel ring in anyway.

It used to be but the RTV bond was enough to stress the glass when it came down in temperature and caused optical aberrations in the airy pattern during star tests.

The threaded holes are for the anchor bolts. Because the back of the mirror is free from the flange any stress in the stainless steel ring is not transmitted to the glass. Stainless steel was chosen for the flange because the primary is very heavy and the anchor points are unfortunately very close to the center of the mirror which makes for a lot of pull on the bolts. It was realistically the only place where they could be located and the threads might have been worn out over time or even fail if it was made out of aluminium.